Ultrapure water
Ultrapure water, high-purity water or highly purified water is water that has been purified to stringent specifications. Ultrapure water is a term commonly used in manufacturing to emphasize the fact that the water is treated to the highest levels of purity for all contaminant types, including organic and inorganic compounds, dissolved and particulate matter, and dissolved gases, as well as volatile and non-volatile compounds, reactive and inert compounds, and hydrophilic and hydrophobic compounds. In contrast to deionized water, UPW has organic particles and dissolved gases removed in addition to ions.
Ultrapure water is typically prepared in three broadly-defined stages: pretreatment, primary treatment, and polishing/transport. While various industries use the term "ultrapure water", the exact definitions differ among industries. Standards for ultrapure water are defined by various groups for the power industry, semiconductor industry, and pharmaceutical industry. Water purity requirements of the semiconductor industry are generally the most stringent to prevent circuit faults at the nanometer scale.
Ultrapure water standards
A number of organizations and groups develop and publish standards associated with the production of UPW. For microelectronics and power, they include Semiconductor Equipment and Materials International , American Society for Testing and Materials International , Electric Power Research Institute , American Society of Mechanical Engineers , and International Association for the Properties of Water and Steam . Pharmaceutical plants follow water quality standards as developed by pharmacopeias, of which three examples are the United States Pharmacopeia, European Pharmacopeia, and Japanese Pharmacopeia.Purification process
Water is typically sourced from city feed water or other local supplies and is taken through a series of purification steps that results in UPW. Some systems recycle used UPW water back into their UPW filtration system as this water is often cleaner than original sources. The purification steps have been broadly categorized into pretreatment, primary treatment, polishing, and/or distribution. These are not strict categories, and certain purification techniques may be present in one or more of the broader steps depending on specific engineering needs or author classification.Pretreatment
Pretreatment produces "purified water" and focuses on removing contaminants with inexpensive methods prior to reverse osmosis or ion exchange during primary treatment. Coagulation and settling are used along with filtration to remove particulate matter that could clog reverse osmosis filters or ion exchange resin beds. Water softening by precipitation may be used for water sources with a relatively high concentrations of dissolved salts to prevent scaling during subsequent steps. The use of coagulation, flocculation, and settling are common in municipal water treatment systems meaning pretreatment may not be necessary depending on locale. For electronics applications, aluminium salts along with lime-based water softeners are used to remove silica during pre-treatment. Transition metal ions like iron and manganese can be removed through oxidation followed by precipitation/flocculation methods.After bulk chemical treatments, pretreatment may include microfiltration or ultrafiltration to remove solids. Ion-exchange resins are commonly used in the pretreament step to further reduce the amount of scale-forming ions like calcium prior to reverse osmosis treatment as scaling can easily clog reverse osmosis membranes.
Primary treatment
Primary treatment aims to remove ions, dissolved gasses, and organic contaminants from pre-treated water. In the 21st century, multiple-pass reverse osmosis is often the primary method used during this step to remove dissolved ions and dissolved organic solids. As a membrane-filtration method it also removes suspended solids as well. Reverse osmosis is often used in this step to remove both dissolved ions and dissolved organic material.Dissolved gases, including oxygen and volatile organic compounds, are removed during primary treatment by vacuum degassing or membrane degassing. Vacuum degassing towers were more common historically, but newer systems have trended towards used of membrane degasification.
Ultraviolet light can used to sterilize purified water during primary treatment though UV treatment can also be left until the polishing stage.
Polishing
Polishing is used in UPW systems to further reduce the already low-level of contaminants present after primary treatment. UV light is often used at this step to sterilize water. Further deionization is conducted using ion exchange beds or electrodeionization. Both inorganic ions and organic ions are removed through these processes. Ion-exchange beds used in the final polishing steps may be non-regenerable in contrast to those used in earlier steps. Ultrafiltration membranes with pore sizes of 0.45 μm are used to remove small particles including bacteria killed by UV sterilization.In semiconductor applications, additional filters with pore sizes ≤200 nm are typically used just before distribution to further reduce particle contamination. Particles must be filtered down to a "critical particle size" that is one-half of the smallest feature size on a semiconductor chip. For example, chips containing a 40 nm features should have all particles >20 nm removed to avoid contamination that prevents computer chips from functioning.
After polishing, UPW is typically cycled continuously through the polishing system to prevent stagnation that can lead to bacterial growth.
Contamination sources and removal
, particles, organic carbon, ions, and dissolved gases are all present in typical municipal water systems and must be removed to create ultrapure water.Particles and bacteria
Particles in UPW can cause defects in semiconductors, especially in photolithographic processes that define nanometer-sized features. Particulates can interfere with etching processes and bridge nanometer-scale features in final circuits causing electrical failures. Particles can be controlled by filtration for larger particles and ultrafiltration for nanometer scale particles. Particle sources can include bacterial fragments or particles from the walls of the fluid handling system.Bacteria have been referred to as one of the most obstinate on this list to control as certain bacteria can still grow, even in low-nutrient environments. Bacteria can be controlled by sanitization or ultrafiltration. Chemical sanitization can be performed using ozone or hydrogen peroxide.
Anions and cations
Cations including sodium, potassium, calcium, and magnesium are common in industrial water supplies. Common anions include chloride, sulfate, and bicarbonate. Several methods are used to remove ions from input water supplies including reverse osmosis, distillation, and/or ion exchange. Distillation can be used to remove non-volatile metal cations and was historically used for water purification. However, distillation is energy-intensive compared to the combination of reverse osmosis and ion exchange which are more common in modern systems.Organic carbon
The removal of organic carbon from water is one of the differentiators between deionized water and ultrapure water. Sources include bacteria, leaching from plastic piping, and dissolved atmospheric sources. Organic carbon can be removed using filtration by activated carbon and oxidation of organic carbon to carbon dioxide/bicarbonate.Dissolved gases
Oxygen dissolved in water can lead to unwanted oxidation of silicon wafers and other materials while gases like carbon dioxide lead to unwanted acidification of water and must be removed. Gases can be removed through various methods including thermal or pressure degassing, membrane degassers or chemical degassing.Silica
Silica naturally leaches from glass walls and enters water supplies. Dissolved silica, in the form of the silicate anion, can be removed through reverse osmosis or anion exchange. Solid, colloidal silica can be removed via ultrafiltration with or without coagulation to increase particle size.Applications
The primary industries using UPW are:- semiconductor devices fabrication process
- solar photovoltaics
- pharmaceuticals
- power generation
- specialty applications such as research laboratories.
Applications in semiconductor industry
UPW is used extensively in the semiconductor industry where the highest grade of purity is required.The use of UPW varies; it may be used to rinse the wafer after application of chemicals, to dilute the chemicals themselves, in optics systems for immersion photolithography, or as make-up to cooling fluid in some critical applications. UPW is even sometimes used as a humidification source for the cleanroom environment.
The primary, and most critical, application of UPW is in wafer cleaning in and after wet etching step during the FEOL stage. Impurities which can cause product contamination or impact process efficiency must be removed from the water during cleaning and etching stage. In chemical-mechanical polishing processes, water is used in addition to reagents and abrasive particles. As of 2002 1-2 parts of contaminating molecules per one million of water ones was considered to be an "ultrapure water".
Water quality standards for use in the semiconductor industry
| Test Parameter | Advanced Semiconductor UPW |
| Resistivity | >18.18 MΩ·cm |
| Total Organic Carbon | <1 μg/L |
| On-line dissolved oxygen | 10 μg/L |
| On-line particles | <200 particles/L |
| Non-Volatile Residue | 100 ng/L |
| Silica | 50 ng/L |
| Metals/Boron | |
| 22 most common elements | <1–10 ng/L |
| Ions | |
| 7 major Anions and ammonium | 50 ng/L |
| Microbiological | |
| Bacteria | <1 CFU/100 mL |
It is used in other types of electronics manufacturing in a similar fashion, such as flat-panel displays, discrete components, hard disk drive platters and solid-state drives NAND flash, image sensors and image processors/ wafer-level optics, and crystalline silicon photovoltaics; the cleanliness requirements in the semiconductor industry, however, are currently the most stringent.